1. Field of the Invention
The present invention relates to a theft monitoring apparatus for protecting merchandise displayed in a shop from shoplifters.
2. Description of Related Art
This type of theft monitoring apparatus includes an antenna for transmitting signals of a specific frequency which is installed at the vicinity of a doorway of a shop, and a receiver for receiving the signals of the specific frequency from the antenna. The receiver, which is referred to as a "tag," is attached beforehand to the merchandise displayed in the shop; if a shoplifter attempts to carry any commodity with the tag out of the shop, then the receiver attached to the commodity detects the signals from the antenna and activates an alarm buzzer or the like to issue warning sound when the shoplifter passes through the doorway. This type of theft monitoring apparatus, however, has been posing a problem in that, if the displayed merchandise includes television sets, personal computers, etc., then the noises emitted from television sets or personal computers unwantedly trigger the receivers attached to the merchandise. To solve the problem, the level of the signal transmitted from the antenna has been set to a relatively higher value than that of the noises, thereby increasing the signal-to-noise ratio.
In recent years, the size of the doorways of shops has been increasing with the increasing size of the shops themselves. With this trend, a plurality of antennas have been installed to cope with the need for expanded areas at the doorways where the commodities can be detected. There is limitation in coping the aforesaid problem by improving the signal-to-noise ratio because the intensity or the like of the signals to be transmitted is limited. There has been proposed a method for measuring the frequency of the signal to be transmitted. A conventional theft monitoring apparatus which includes a plurality of antennas installed to detect the frequency of a transmitted signal will be explained in conjunction with FIG. 5 through FIG. 9.
FIG. 5 is a conceptual diagram illustrating the transmitting unit of a conventional theft monitoring apparatus; FIG. 6 is a block diagram showing a transmitter; FIG. 7 is a diagram illustrating the waveform of a signal transmitted from an antenna of the transmitting unit; FIG. 8 is a block diagram illustrating a receiver; and FIG. 9 is another conceptual diagram illustrating the transmitting unit. In this example, a case wherein two antennas are installed will be explained.
In FIG. 5, antennas 1 and 2 of the transmitting unit are installed, for example, at both sides of a doorway of a shop. Transmitters 3 and 4 are connected to the feeders, not shown, of the antennas 1 and 2, respectively. The transmitters 3 and 4 share the same configuration; they have a signal generator 5, a timing controller 6, and a driver 7 as shown in FIG. 6. The signal generator 5 has a sine wave oscillator circuit (not shown) which oscillates at a frequency of 64 kHz (hereinafter referred to as "carrier wave"). The oscillation signal issued from the signal generator 5 is supplied intermittently to the driver 7 by a control signal generated by the timing controller 6, then it is amplified as necessary by the driver 7 before it is emitted through the antenna 1 or 2. The control signal generated by the timing controller 6 is composed of, for example, successive pulses which have a nominal repetition frequency of 1000 Hz and which turn ON/OFF at a 50% duty cycle (hereinafter referred to as "modulated wave"); during the ON period, the carrier wave issued from the signal generator 5 is supplied to the driver 7 and emitted in a burst mode through the antennas 1 or 2, whereas during the OFF period, the carrier wave is not applied to the driver 7, so that it is not emitted through the antennas 1 or 2. The carrier wave of 64 kHz radiated through the antennas during the ON period of the modulated wave is referred to as a "burst signal."
FIG. 7A illustrates the waveform of the signal which is, for example, transmitted from the antenna 1, and FIG. 7B illustrates the waveform of the signal transmitted from the antenna 2. In this diagram, T1 indicates the period of time of one cycle which is determined by the repetition frequency 1000 Hz of the modulated wave; in a half cycle (T1/2), the carrier wave is radiated for about 16 cycles and turns into the burst signal A. The period during which the burst signal A is radiated corresponds to the ON period of the modulated wave. Likewise, in the case shown in FIG. 7B, a burst signal B is radiated in a half cycle (T2/2). Thus, the burst signal A or B is transmitted in succession in the cycle T1 or T2; however, since the transmitters 3 and 4 are provided separately, the cycles T1 and T2 do not necessarily fully coincide, and the frequencies of the carrier wave (64 kHz) in the burst signals A and B do not necessarily fully coincide, either. Furthermore, times ts1 and ts2 at which the burst signals A and B rise or start, respectively, do not completely agree; there is usually a time difference ts.
The receiver is constituted by a receiving antenna 8 making up a resonance circuit, a detector 9, a waveform shaper 10, a determiner 11, and an alarm 12 as shown in FIG. 8. The receiving antenna 8 receives the transmitter signals shown in FIG. 7A and FIG. 7B, and amplifies them as necessary before detecting them so as to envelope-detect the carrier waves of the burst signals A and B shown in FIGS. 7A and 7B, thereby extracting the modulated wave of the 1000 Hz repetition frequency. The modulated wave is then supplied to the waveform shaper 10 which shapes it into complete pulses. The determiner 11 determines that the received signal is the modulated wave of the 1000 Hz repetition frequency from the half cycle (T1/2) or (T2/2) during the ON period of the pulse, thereby actuating the alarm 12 to issue the warning sound.
FIG. 9 is a conceptual diagram showing the transmitting unit of another conventional example which has only one transmitter. The transmitter signals sent from the antennas 1 and 2 have only the waveform illustrated in FIG. 7A, and the frequencies and phases (accordingly the cycles) of the carrier waves and the modulated waves are completely identical. There is no difference in the rising time of the burst signals; however, a cable 12 is required to interconnect the antennas 1 and 2.
In the case of the theft monitoring apparatus shown in FIG. 5, the burst signal A or B present in the period of the half cycle (T1/2) or (T2/2) is received, and the transmitter signals sent out from the antennas 1 and 2 are set at a high level; therefore, the theft monitoring apparatus is unlikely to be accidentally actuated by the noises of personal computers, etc. Further, if a commodity with a tag attached thereto is passed in the vicinity of either the antenna 1 or 2, e.g. the antenna 1, then the transmitter signal level of the other antenna, e.g. the antenna 2, which is farther from the commodity is lower, enabling only the transmitter signal of a higher level transmitted from the nearby antenna 1 to be received; hence, no problem arises.
If, however, the commodity with the tag thereon is passed at the midpoint between the two antennas 1 and 2, then the receiver receives the transmitter signals of approximately the same level from the two antennas 1 and 2. In such a case, the carrier waves received by the receiver are likely to interfere with each other to cancel their levels and disable the tag, because the transmitter signals sent out from the antennas 1 and 2 do not necessarily coincide phase-wise as mentioned above, that is, the cycles T1 and T2, the half cycles (T1/2) and (T2/2) during which the burst signals A and B are present, the start time ts1 and ts2, and the frequencies of the carrier waves in the burst signals A and B do not always agree. If the burst signal A from the antenna 1 is fully shifted by a half cycle time-wise from the burst signal B from the antenna 2 and the carrier waves are radiated in succession, then even if the carrier waves are received by the receiver, the determiner will not be able to identify the modulated wave of the 512 Hz repetition frequency after detecting them.
The problem mentioned above would be solved by making the carrier wave and the modulated wave of the transmitter 3 connected to the antenna 1 completely synchronized with the carrier wave and the modulated wave of the transmitter 4 connected to the antenna 2; however, doing so would complicate the configuration of the transmitters and add to the cost of the entire apparatus. The system shown in FIG. 9 does not have such a problem since it has only one transmitter 3; however, it would require the cable 12 for connecting the antennas 1 and 2 to be extended in a large shop, making it disadvantageous in the aspect of the installation of the system, the appearance, or the maintenance and inspection after installation.